I was reading about the Paleolithic Diet and I came across this study ( http://www.thepaleodiet.com/articles/J%20Nutr%20Environ%20Med%202003.pdf ) from the Journal of Nutritional and Environmental Medicine that suggests avoiding dairy and in particular references a study on rats that showed casein (and soy protein) decreased insulin sensitivity.
I know that is only a study on rats, but does anyone know any more about this or if anyone has done human studies to measure the same thing? It seems like that is an area of research which would have been done already, so hopefully someone can shed some light on the topic.